Photosynthesis rate graphs

A photosynthesis rate graph shows how the rate changes as one factor, such as light intensity, changes. The factor being varied is plotted on the horizontal axis; the rate of photosynthesis is plotted on the vertical axis. Other factors should be kept constant so that the effect of the chosen factor can be examined.

Plotting a photosynthesis rate graph

Label each axis with the quantity and its unit. For example, the horizontal axis could show light intensity in lux, and the vertical axis could show the volume of oxygen produced in $$\mathrm{cm^3/min}$$.

Choose a consistent scale for each axis that covers all the data and uses the available space appropriately. For each pair of measurements, find the factor value on the horizontal scale and the corresponding rate on the vertical scale. Plot a point where the two positions meet.

Draw an appropriate line or curve to show the pattern. It should follow the overall trend of the points; it does not have to pass through every point if the measurements vary.

For example, suppose the data include a light intensity of $$200\ \mathrm{lux}$$ and a rate of $$4\ \mathrm{cm^3/min}$$. Plot the point at $$200$$ on the horizontal scale and $$4$$ on the vertical scale. To read that point from the graph, reverse the process: read its horizontal and vertical positions using the axis scales.

Interpreting the pattern

Read the graph from left to right to see what happens to the rate as the factor increases:

• An upward line or curve shows that the rate rises.
• A plateau, or level section, shows that further increases in the plotted factor do not increase the rate.
• A downward line or curve shows that the rate falls.

A plateau may indicate that another factor is limiting photosynthesis. Increasing the factor on the horizontal axis then has no further effect because the other factor restricts the rate.